The present invention pertains to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission display screens.
Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This requires the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pagers, cell phones, pocket-TVs, personal digital assistants, and portable electronic games.
One problem associated with the FEDs is that the FED vacuum tubes may contain minute amounts of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes, focus electrodes, (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube.
In addition, electron emission from the emitter electrodes to the gate electrodes can cause both emitter and gate degradation. For instance, the gate is positive with respect to the emitter causing an attraction of electrons from the emitter electrodes to the gate electrodes. In addition, the presence of the high pressure facilitates electron emission from emitters to gate electrodes. The result is that some electrons may strike the gate electrodes rather than the display screen. This situation can lead to gate electrode degradation including overheating of the gate electrodes. The emission to the gate electrodes can also affect the voltage differential between the emitters and the gate electrodes. Electron emission from the emitter electrodes to the gate electrodes can also cause ions and other material debris to be released from the gate and thereby become attached to the emitter electrode. This can cause emitter degradation.
It is worth noting that electrons may also hit spacer walls and focus electrodes, causing non-uniform emitter degradation. Problems occur when electrons hit any surface except the anode, as these other surfaces are likely to be contaminated and out gas because they are not scrubbed by the electron beam during normal tube operation.
In addition, as the electrons jump the gap between the electron-emissive elements and the gate electrode, a luminous discharge of current may also be observed. Severe damage to the delicate electron-emitters may also result. Naturally, this phenomenon, generally known as xe2x80x9carcing,xe2x80x9d is highly undesirable.
Conventionally, one method of avoiding the arcing problem is by manually scrubbing the FED vacuum tubes to remove contaminant material. However, it is difficult to remove all contaminants with that method. Further, the process of manual scrubbing is time-consuming and labor intensive, unnecessarily increasing the fabrication cost of FED screens.
Accordingly, an embodiment of the present invention provides an improved method of removing contaminant particles from the FED screen. The present invention also provides for an improved method and circuit of operating field emission displays to prevent gate-to-emitter currents during turn-on and turn-off thereby reducing potential gate and emitter electrode degradation. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
Embodiments of the present invention provide for a method of removing contaminant material in newly fabricated field emission displays. According to one embodiment of the present invention, contaminant particles are removed by a conditioning process, which includes the steps of: a) driving an anode of a field emission display (FED) to a predetermined voltage; b) slowly increasing an emission current of the FED after the anode has reached the predetermined voltage; and c) providing an ion-trapping device for catching the ions and contaminants knocked off by emitted electrons. In this embodiment, by driving the anode to the predetermined voltage and by slowly increasing the emission current of the FED, contaminant species are effectively removed without damaging the FED.
Embodiments of the present invention also provide for a method and circuit for operating FEDs to prevent gate-to-emitter current during turn-on and turn-off. This embodiment protects against emitter and gate degradation during FED operation. In this embodiment, the method includes the steps of: a) enabling the anode display screen; and, b) enabling the electron-emitters a predetermined time after the anode display screen is enabled. In this embodiment, by allowing sufficient time for the anode display screen to reach a predetermined voltage before the emitter is enabled, the emitted electrons will be attracted to the anode. In this way, gate-to-emitter current, gate to spacer current, and gate to focus current are effectively eliminated when an FED is turned on. In the present embodiment, the anode display screen is enabled by applying a predetermined high voltage to the display screen, and the electron-emitters are enabled by driving appropriate voltages to the gate electrodes and emitter electrodes of the FED.
In yet another embodiment of the present invention, the method of operating field emission displays to prevent gate-to-emitter current includes the steps of: a) disabling the emitters for a predetermined time; and, b) disabling the anode display screen after the electron-emitters are disabled. In this embodiment, by allowing sufficient time for the electron-emitters to be disabled before disabling the anode display screen, all remaining electrons will be attracted to the anode. In this way, gate-to-emitter current is eliminated during a turn-off sequence of the FED. In the present embodiment, the anode display screen may be disabled by removing the voltage source from the anode and allowing it to be at ground potential, and the electron-emitters are disabled by driving the gate electrodes and the emitter electrodes to the ground voltage.
In yet another embodiment, the present invention includes a circuit and method for turning-on and turning-off elements of a field emission display (FED) device to protect against emitter electrode and gate electrode degradation. The circuit includes control logic having a sequencer which in one embodiment can be realized using a state machine. Upon power-on, the control logic sends an enable signal to a high voltage power supply that supplies voltage to the anode electrode. At this time a low voltage power supply and driving circuitry are disabled. Upon receiving a confirmation signal from the high voltage power supply, the control logic enables the low voltage power supply which supplies voltage to the driving circuitry. Upon receiving a confirmation signal from the low voltage power supply, or optionally after expiration of a predetermined time period, the control logic then enables the driving circuitry which drives the gate electrodes and the emitter electrodes which make up the rows and columns of the FED device. Upon power down, the control logic first disables the low voltage power supply, then the high voltage power supply. The above may occur each time the FED is powered-on and powered-off during the normal operational use of the display. By so doing, embodiments of the present invention reduce emitter electrode and gate electrode degradation by restricting electron emission from the emitter electrode directly to the gate electrode, the focus electrode or the spacers.
Embodiments of the present invention include the above and further include a method of operating a field emission display, the method comprising the steps of: providing the field emission display with electron-emissive elements for emitting electrons, a gate electrode for controlling electron emission from the electron-emissive elements, and a display screen for collecting the electrons; enabling the display screen to establish a voltage differential between the display screen and the electron-emissive elements; and following enabling of the display screen, enabling the gate electrode by delaying substantial electron emission from the electron-emissive elements until the voltage differential has been established to direct the electrons towards the display screen and to substantially prevent the electrons from striking the gate electrode.
Embodiments of the present invention further include a field emission display device comprising: a baseplate; a plurality of electron-emissive elements on the baseplate; a gate electrode on the baseplate for controlling electron emission from the electron-emissive elements; a display screen spaced from the baseplate and configured for collecting electrons emitted from the electron-emissive elements to generate an image thereon; and a control circuit configured to control a flow of electrons to the electron-emissive elements, the control circuit allowing a voltage differential to be established between the display screen and the electron-emissive elements prior to substantial electron emission from the electron-emissive elements to prevent substantial gate-to-emitter current during turn on of the field emission display device.
Embodiments also include a field emission display device comprising: a display screen comprising: rows and columns of; and an anode electrode, wherein each of the pixels comprises respective emitter electrodes and respective gate electrodes that are controlled by driver circuitry; a high voltage power supply coupled to provide a high voltage to the anode electrode and coupled to receive a first enable signal, the high voltage power supply also for generating a confirmation signal upon reaching its operational voltage; a low voltage power supply coupled to provide a low voltage to the driver circuitry and coupled to receive a second enable signal; and control logic coupled to the high and low voltage power supplies and also coupled to the driver circuitry, the control logic, in response to a power-on signal, for powering-on the display screen by generating the first enable signal and then generating the second enable signal in response to the confirmation signal to prevent electron emission from the emitter to the gate electrodes.
Embodiments include the above and wherein the driver circuitry is coupled to receive a third enable signal and wherein the control logic is also for enabling the driver circuitry by generating the third enable signal after enabling the low voltage power supply. Embodiments include the above and wherein the control logic is also for powering-down the display screen by first disabling the low voltage power supply and then by disabling the high voltage power supply. Embodiments include the above and wherein the control logic is realized by a state machine sequencer and further comprising a gas-trapping device to trap contaminants within the display screen.